1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing device that employs a heating member such as a fixing heater that is heated by a charged power of a charging element.
2. Description of the Related Art
A technique for improving a power-saving effect is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-315567, Japanese Patent Application Laid-Open No. 2002-174988, and Japanese Patent Application Laid-Open No. 2003-140484, in which a sudden transient build-up current can be carried to a heating member (a fixing heater) of a fixing device employed in an electrophotographic image-forming apparatus using not only power supplied from a commercial power supply but also that supplied from a chargeable auxiliary power-supply using an electric double-layer capacitor or the like.
According to the above technique, if a mass-storage capacitor is used as the auxiliary power supply, a high current can be instantly supplied to the fixing device even if the supply of the power from the commercial power supply to the fixing device runs short. It is, therefore, possible to prevent degradation of fixability of the fixing device due to the power shortage.
Meanwhile, if an alternating current (AC) is mainly used as the power supplied from the commercial power supply to the fixing heater, an inrush current is often generated when the fixing device is subjected to temperature control. As a result, reliability of the fixing device is deteriorated. Furthermore, to suppress the inrush current, there is known soft-starting of application of a current to an AC control element such as a triac synchronously with a phase angle of the commercial power supply. The soft-starting has, however, a disadvantage of generating a conducted interference with the commercial power supply.
To cope with the disadvantage, a technique is disclosed in, for example, Japanese Patent Application Laid-Open No. H9-218720, Japanese Patent Application Laid-Open No. H11-109786, and Japanese Patent No. 3359141 (Japanese Patent Application Laid-Open No. H7-219655), in which the AC from the commercial power supply is rectified into a pulsating current, and the pulsating current is applied to the fixing heater while power-controlling the pulsating current at a frequency higher than a frequency of the AC of the commercial power supply.
With this mechanism, a load current is carried over entire cycles of the AC and a power factor of the commercial power supply is improved. In addition, by changing an amplitude of a voltage output to the fixing heater, a peak of the load current can be made proportional to load power. It is, therefore, possible to employ a switching element having an optimum current capacity to correspond to the load power.
However, the technique disclosed in Japanese Patent Application Laid-Open No. H9-218720, Patent Application Laid-Open No. H11-109786, and Japanese Patent No. 3359141 (Japanese Patent Application Laid-Open No. H7-219655) is unable to overcome the power shortage solved by the Japanese Patent Application Laid-Open No. 2000-315567, Patent Application Laid-Open No. 2002-174988, and Patent Application Laid-Open No. 2003-140484, and as a result, the power shortage may cause a degradation in the fixability of the fixing device.
Moreover, according to the conventional techniques, the power of the commercial power supply and that of the auxiliary power supply constituted by the battery element should be supplied to different fixing heaters. As a result, the image forming apparatus is disadvantageously complicated and expensive.
Furthermore, even if the power of the commercial power supply and that of the auxiliary power supply are supplied to one fixing heater, the power is supplied only from the power supply having a high supply voltage. As a result, to quickly activate the fixing heater, a commercial power supply having a high power capacity or an auxiliary power supply is necessary.